A basal medium incorporating defined nutrients in place of yeast extract was devised to facilitate further investigations of nutritional requirements of Entamoeba histolytica. Addition of bile extract to TYI-S-33 medium was essential for growth of one strain of Giardia lamblia and contributed to support of three other strains. A neonatal mouse model was developed for study of G. lamblia infections. Axenically cultivated trophozoites inoculated intragastrically through the abdominal wall produced infections which persisted for three weeks and were accompanied by cyst formation. Drastic modification of a cooling-rate schedule widely used for freeze-preservation of cells led to increased recoveries of E. histolytica stored in liquid nitrogen vapors. A simple inexpensive freezing device was highly effective for cryopreservation of E. histolytica, G. lamblia and Trichomonas hominis. Significant reductions in serum iron, total iron-binding capacity, and percent saturation of transferrin were found in 50 adult male patients acutely ill from amebic liver abscess in a Mexico City hospital study. These findings indicate that the disease is accompanied by changes in iron status similar to those in patients with acute bacterial or fungal infections. Studies to develop a means of inducing encystation of E. histolytica in axenic cultures were initiated.